The present invention relates to a sewer system in which sewage is conveyed along a main trunk line from a plurality of sewage sources to a destination point, and more particularly, to a sewage system having a generally horizontal trunk line in which the flow of sewage along the trunk line is maintained despite the absence of an elevation gradient along the path of sewage flow and in which the back flow of sewage from the trunk line to the sources is substantially inhibited without requiring check valves to be placed in the path of sewage flow.
In the past, systems for conveying sewage have generally required an elevation gradient to be present along the path of sewage flow in order for the flow of sewage to be maintained. This can be easily accomplished where the terrain of the land in which the sewer system is located slopes downwardly toward a predetermined destination point where the sewage is disposed of, processed, or the like. However, where the terrain is substantially horizontal or slopes slightly upwardly, conventional gravity systems can not maintain sewage flow.
In one known prior art type of sewer system, such as illustrated in U.S. Pat. No. 2,012,495, the sewer pipe line is constructed with an elevation gradient despite the absence of such a gradient in the terrain of the land in which the sewer system is located. This is accomplished by successively placing the sewer line at greater depths underground along the flow path of the sewage, to provide a downhill flow path. This construction can only be used for sewer systems of relatively short lengths, since the costs of digging and laying the sewer line will become prohibitive as the length of the sewer line increases, due to the requirement of going deeper into the ground.
Other prior art attempts at solving this problem are illustrated in U.S. Pat. Nos. 3,211,167 issued to Clift et al, and 3,730,884, issued to Burns et al. In the Clift patent a pump is provided at each source of sewage, e.g. dwelling, to pump sewage from the dwelling into a main pipe line and thereby induce sewage flow in the pipe line. The costs of providing a pump in each dwelling and maintaining such a large number of pumps greatly reduces the feasibility of such a system. In the system of the Burns patent, a subatmospheric pressure is provided at the downstream end of each of a plurality of sewer pipe lines. The cost of producing a vacuum in a plurality of pipe lines and the vulnerability of such a system to malfunction, due to cracks in a pipe or the like, may render such a vacuum system impractical.
Another problem quite common to sewage systems is that of back flow of sewage. When a sewage flow path becomes clogged, the sewage has a tendency to flow in a reverse direction towards the sewage sources located immediately upstream of the clogged point, due to the flow of sewage from sources further upstream. Therefore, it becomes necessary to provide check valves at various points along the main pipe line of the sewer system as well as within the various feeder lines leading from the sewage sources to the main pipe line. Due to the nature of the materials flowing in the sewer system, the check valves themselves can quite easily become clogged, and therefore it is necessary to provide manholes or other means of access to the various check valves, further increasing the cost of the sewer system both in terms of initial construction and maintainance.